Examples of safety valves useful for downhole operations are disclosed in U.S. Pat. No. 5,050,839 issued to Dickson et al. The entire disclosure of U.S. Pat. No. 5,050,839 is incorporated herein by reference. The assignee of U.S. Pat. No. 5,050,839 is also the assignee of the present application.
Each of the safety valves disclosed in U.S. Pat. No. 5,050,839 utilizes a valve assembly comprising: a valve housing; a ball assembly which is reciprocatably positionable in the valve housing; and a pair of ball assembly control arms which are held in fixed position in the valve housing. The ball assembly includes: a valve seat having a passageway extending therethrough from the top of the valve seat to the bottom of the valve seat; a ball valve rotatably positioned adjacent the bottom of the valve seat for selectively sealing the valve seat passageway; a pair of control arms connected between the valve seat and the ball valve such that (a) the control arms hold the ball valve adjacent the bottom of the valve seat and (b) the connections between the control arms and the ball valve define an axis of rotation for the ball valve; a first coupling disc connected to the ball valve and positioned between the ball valve and one of the control arms; and a second coupling disc connected to the ball valve and positioned between the ball valve and the other control arm. Each of the control frames used in the Dickson et al. valve assembly provides a single stationary lug member. The stationary lug member of one control frame projects into an aperture formed in one of the coupling discs while the stationary lug member of the other control frame projects into an aperture formed in the other coupling disc. The two stationary lug members are directly opposed to each other and lie outside of the axis of rotation of the ball valve. Consequently, when the ball assembly is reciprocated in the valve housing with respect to the stationary lug members, the stationary lug members cause the ball valve to rotate. Thus, by causing the ball assembly to reciprocate in the valve housing, the Dickson et al. safety valve can be opened and closed.
In FIGS. 1 and 2 of U.S. Pat. No. 5,050,839, Dickson et al. disclose a subsurface safety valve having a design directed toward preventing the occurrence of galling between the valve seat and the ball valve when the ball valve is rotated from its closed position to its open position. As explained by Dickson et al., valve galling tends to occur when a ball valve is rotated from closed position to open position due to the fact that, when the ball valve is in closed position, the high relative formation pressure acting on the bottom of the ball valve urges the ball valve strongly against the valve seat. In the subsurface safety valve depicted by Dickson et al. in FIGS. 1 and 2, the coupling discs used in the Dickson et al. valve assembly are designed such that, as the ball valve is rotated from its closed position to its open position, the control discs tend to urge the ball valve away from the valve seat.
In FIGS. 10 and 11 of U.S. Pat. No. 5,050,839, Dickson et al. disclose a safety valve design which is particularly well suited for use in a subsurface test tree. In this embodiment, the coupling discs used in the valve assembly are designed such that, as the ball valve is rotated about its axis of rotation toward its closed position, the coupling discs of the Dickson et al. valve assembly tend to urge the ball valve toward the valve seat. Consequently, as the ball valve is rotated to its closed position, the ball valve can readily cut a wire or reeled tubing which has been extended through the safety valve.
As will be appreciated by those skilled in the art, subsurface safety valves of type disclosed in U.S. Pat. No. 5,050,839 have a significant shortcoming. This shortcoming is substantially alleviated by the present invention. In a safety valve of the type disclosed in U.S. Pat. No. 5,050,839, the control arms linking the valve seat and the ball valve must provide sufficient play between the ball valve and the valve seat to allow the ball valve to be rotated. As indicated above, when the ball valve is closed, the formation pressure acting on the bottom of the ball valve will normally be substantially greater than the tubing pressure acting on the top of the ball valve. The resulting pressure differential urges the ball valve tightly against the bottom of the valve seat such that the safety valve is prevented from leaking. However, if the tubing pressure acting on the top of the closed ball valve is caused to exceed the formation pressure, the resulting pressure differential urges the ball valve away from the bottom of the valve seat such that the high pressure fluid in the tubing above the ball valve is allowed to flow past the ball valve and into the formation.
If the tubing pressure acting on the top of the ball valve is substantially greater than the formation pressure, the resulting amount and rate of fluid leakage into the formation can be sufficient to damage the formation. For example, as will be understood by those skilled in the art, when a subsea test tree containing a safety valve of the type in question is "unlatched," the ocean water above the safety valve may suddenly exert a tremendous amount of hydrostatic pressure on the top of the ball valve. The hydrostatic pressure exerted on the top of the ball valve urges the ball valve away from the valve seat such that a large amount of ocean water can be allowed to flow into the formation.